This invention relates to fluid collection devices, and more particularly, to blood collection devices having means for partitioning the relatively light phase from the relatively heavy phase.
When taking blood samples for test purposes, whole blood is generally drawn into an evacuated collection tube and the tube subsequently centrifuged to separate the blood into its relatively light phase, serum or plasma, and its heavier cellular phase. Blood phase separators or partitioning devices have been used to provide a partition or barrier between the separated phases until the light phase is removed for clinical testing. Many types of blood phase partitioning devices have been proposed, all with varying degrees of success. Some proposed partitioning arrangements include the use of a sealant or gel-like thixotropic material having a specific gravity intermediate the specific gravities of the light and heavy blood phases so that during centrifugation and phase separation the sealant flows to the interface of the two phases and forms a partition between them. Various gel-like thixotropic materials or sealants are now well known. For example, in U.S. Pat. No. 3,852,194, a mixture of silicone and hydrophobic silicon dioxide powders is used to form a partition between the separated phases. In U.S. Pat. Nos. 4,021,340; 4,088,582 and 4,055,501, mixtures including liquid polybutene polymer and silicon dioxide powders are used as phase partitioning materials.
A serious problem in utilizing such partitioning materials is that the partitioning barrier is sometimes formed too soon after centrifugation begins so that blood cells are trapped above the partitioning barrier in contact with the serum or plasma after phase separation. Such cells tend to contaminate the lighter phase to such a degree, in some cases, that certain test results are unreliable or inaccurate. Cells have been trapped in or above the partitioning barrier because the rising partitioning material engages descending cells and carries them to the interface of the two phases.
Also, in some cases, the separated serum was found to have excessive lactic dehydrogenase (LDH) which was believed to be caused by hemolysis of red cells due to the impact of the cells with the partitioning material under the influence of centrifugal forces. When this occurs, the test results are inaccurate since some of the LDH indicated is caused by the process, that is, due to centrifugal separation and partitioning of the phases.